This invention is generally directed to liquid developer compositions and, more specifically, to liquid developers containing block polymer negative charge directors comprised of a total of at least three blocks, ammonium A blocks and nonpolar B blocks in various combinations. In embodiments, the charge directors of the present invention are comprised of triblock copolymers of the formula A-B-A wherein the polar A block is an ammonium containing segment and B is a nonpolar block segment which, for example, provides for charge director solubility in the liquid ink fluid like ISOPAR.TM., and wherein the A blocks have a number average molecular weight range of from about 200 to about 120,000; the B blocks have a number average molecular weight range of from about 2,000 to 190,000; the ratio of M.sub.w to M.sub.n of the triblock copolymer A-B-A is 1 to 5; and the total number average molecular weight of the ABA copolymer is, for example, from about 6,200 to about 300,000, and preferably about 200,000. The developers of the present invention can be selected for a number of known imaging and printing systems, such as xerographic processes, wherein latent images are rendered visible with the liquid developers illustrated herein. The image quality, solid area coverage and resolution for developed images usually require sufficient toner particle electrophoretic mobility. The mobility for effective image development is primarily dependent on the imaging system selected. The electrophoretic mobility is primarily directly proportional to the charge on the toner particles and inversely proportional to the viscosity of the liquid developer fluid. A 10 to 30 percent change in fluid viscosity caused, for instance, by a 5.degree. C. to 15.degree. C. decrease in temperature could result in a decrease in image quality, poor image development and background development, for example, because of a 5 percent to 23 percent decrease in electrophoretic mobility. Insufficient particle charge can also result in poor transfer of the toner to paper or other final substrates. Poor or unacceptable transfer can result in, for example, poor image developer solid area coverage when insufficient toner is transferred to the final substrate and can also lead to image defects such as smears and hollowed fine features. To overcome or minimize such problems, the liquid toners of the present invention were arrived at after substantial research efforts, and which toners result in, for example, sufficient particle charge for image transfer and wherein the developer mobility is maintained within the desired range of the particular imaging system employed. Examples of specific advantages associated with the present invention include increasing the desired negative charge on the developer particles and in embodiments providing a charge director, that is superior since, for example, it provides higher charging, lower conductivity, and allows for lower costs primarily because lower concentrations of charge director are effective as compared to similar AB diblock charge directors like AB tetraalkyl quaternary ammonium block copolymers, AB diblock protonated ammonium salts or zwitter ion containing copolymers, lecithin, and metal salts of petroleum fractions. Some of the aforementioned additives like lecithin contain impurities which can have an adverse effect on their intended function. The aforementioned superior charge can result in improved image development and superior image transfer. The multiple block nature of the invention charge directors is believed to allow for more efficient micelle formation which enables higher particle charging.
Examples of acceptable conductivity and mobility ranges for developers charged with the ammonium salt and zwitterionic triblock copolymer charge directors of the present invention are as illustrated herein. Conductivities, measured at ambient temperature (21.degree. C. to 23.degree. C.), for developers containing one percent toner solids are considered to be in the high range at 14 to 100 pmhos/centimeters. Medium conductivities are from about 6 to about 13 pmhos/centimeters and low conductivities are from 0.1 to about 6 pmhos/centimeters. As conductivities increase into the undesirable high range, excess ions can compete with toner particles of the same charge for development of the latent image giving rise to low developed mass resulting in low print density images. Also, with a low to medium conductivity of less than 14 pmhos/centimeter, the liquid toner or developer of this invention can possess a mobility of between about -1 to 1.99.times.10.sup.-10 m.sup.2 /Vs and preferably -2.00 to 2.49.times.10.sup.-10 m.sup.2 /Vs, and most preferably -2.50 to 5.times.10.sup.-10 m.sup.2 /Vs. Furthermore, it is desirable that these mobility ranges occur within about 10 days and preferably within 2 days of adding the charge director to the liquid toner.
A latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. The aforementioned dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, it is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet.
Liquid developers can comprise a thermoplastic resin and a dispersant nonpolar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present in the developer. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 10 microns (.mu.m) average by area size as measured using the Horiba Capa 500 or 700 particle sizer.
Since the formation of images depends, for example, on the difference of charge between the toner particles in the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and charge adjuvants which increase the magnitude of the charge, such as polyhydroxy compounds, amino alcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, and the like to the liquid developer comprising the thermoplastic resin, the nonpolar liquid and the colorant.
U.S. Pat. No. 5,019,477, the disclosure of which is totally incorporated herein by reference, discloses a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors disclosed may include both negative charge directors such as lecithin, oil-soluble petroleum sulfonate and alkyl succinimide, and positive charge directors such as cobalt and iron naphthanates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyl toluene and styrene, and (iv) selected from the group consisting of butadiene and acrylate.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles of resin and opitonal charge adjuvant may contain pigment particles, wherein the resin can be selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof, and in embodiments thermoplastics generally. The liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
U.S. Pat. No. 5,026,621 discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) of a fluorine-free vinyl or olefin monomer block unit. The functional segment of the block copolymer is oriented to the surface and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner so that the toner is provided with both liquid repelling and solvent soluble properties.
Moreover, in U.S. Pat. No. 4,707,429, the disclosure of which is totally incoporated herein by reference, there are illustrated, for example, liquid developers with an aluminum stearate charge additive. Liquid developers with charge directors are illustrated in U.S. Pat. No. 5,045,425. Additionally, of interest are U.S. Pat. Nos. 4,760,009; 5,034,299 and 5,028,508.
In copending patent application U.S. Ser. No. 986,316 the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle, which developer has a melting point of at least about 25.degree. C., the contact occurring while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.
In copending patent applications and patents and U.S. Ser. No. 065,414 U.S. Pat. Nos. 5,306,591 and 5,308,731, the disclosures of which are totally incorporated herein by reference, there is illustrated the following: a liquid developer comprised of a certain nonpolar liquid, thermoplastic resin particles, a nonpolar liquid soluble ionic or zwitterionic charge director, and a charge adjuvant comprised of an aluminum hydroxycarboxylic acid, or mixtures thereof; U.S. Pat. No. 5,306,591 discloses a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and U.S. Pat. No. 5,308,731 discloses a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid.
Illustrated in copending applications U.S. Ser. No. 200,988 is a positively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and a charge adjuvant comprised of a polymer of an alkene and unsaturated acid derivative; and wherein the acid derivative contains pendant ammonium groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment; and. U.S. Ser. No. 204,012 is a negatively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and an insoluble charge adjuvant comprised of a copolymer of an alkene and an unsaturated acid derivative, and wherein the acid derivative contains pendant fluoroalkyl or pendant fluoroaryl groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment.